1. Field of Invention
The invention relates to a MOSFET and the method for manufacturing them. In particular, it relates to a MOSFET for ink-jet heads and the associated manufacturing method.
2. Related Art
The inkjet printer ejects ink droplets on a printing medium to form desired texts or pictures. The size, shape, material, density, and positioning of the produced ink droplets are important factors that will affect the printing quality. The smaller the ink droplet size is, the higher printing resolution one can achieve. However, under normal conditions, the printing speed is also sacrificed. In order to promote the printing speed and resolution, the number of nozzles on the print head chip is the substantial solution.
To achieve this objective, driving devices with switch and active characteristics such as transistors and the ink droplet actuators are often integrated into a single inkjet print head chip. The number of cartridge contact points X and the number of nozzles Y are promoted from the one-to-one driving mode (X=Y) to one-to-many mode (Y=(X/2)2). Such integrated driver head (e.g. ejecting ink droplets using thermal bubbles) is normally made by connecting a metal oxide semiconductor field effect transistor (MOSFET) with a droplet actuating thermal resistor in series. A sufficiently high driving current has to pass through the MOSFET at a voltage around 10V for the thermal bubble actuator to obtain enough energy to generate bubbles that eject ink droplets out. Since the MOSFET used in the inkjet print head chip is used under large currents and voltages, it is commonly to use with phosphosilicate glass (PSG) for the 1st interlayer dielectric. The temperature is raised to be higher than 1000° C. to drive the phosphor within the PSG into silicon, forming the N+ diffusion of source and drain with deep junctions. The junction depth is about 1.2 μm to 1.8 μm. In addition to avoiding the spiking at the Al—Si contact surface, the N+ diffusion with deep junction can increase the breakdown voltage of the MOSFET. With the increase of the printing speed and resolution, using tiny ink droplets and increasing the inkjet frequency have become the primary objectives. Consequently, the energy required to form each individual ink droplet gets lower. The needed current and voltage also decreases accordingly. At the same time, the line width required by the MOSFET is made thinner to increase the device density on a chip. In summary, these requirements of inkjet print head chips have made the existing driving device structure and specification are not satisfactory.